I. Field of the Invention
The present invention relates generally to mobile telephone systems, and more particularly to a system and method for control of the output power for an outdoor unit coupled to a wireless device transferring an information signal, through the use of a closed loop circuit. The present invention is most applicable to wireless communications devices used in communication systems using code division multiple access (CDMA) modulation techniques, where output power control is critical.
II. Related Art
Mobile telephone systems allow customers to establish communication links or place telephone calls from wireless devices such as portable or hand held mobile phones. Calls initiated or received by wireless devices used in such systems are processed by a wireless network. One type of wireless network is a terrestrial cellular communication system communicating via a series of base stations and ground-based antennas that operate in the 800-1900 MHz range. Cellular communication systems limit the user to communication within a cell, which comprises a geographical service area to which the base station antennas can transmit. Users can move from cell to cell through known hand-off procedures that transfer calls from one cell to another. However, if no base station is within range of the mobile transmitter, such as in a rural area, a user cannot use the mobile telephone service.
Developments in mobile telephone system technology have led to wireless communication systems or networks that can transfer signals using a Low Earth Orbit (LEO) satellite system. The satellite systems can transmit and receive signals in rural areas as well as cities through the beams they project, and a user does not need to be within close range of a ground-based antenna. As a result, satellite communication systems are not limited to major cities as are cellular networks. In addition, each LEO satellite is capable of carrying a large number of user transmissions simultaneously. Various satellite access schemes such as time division multiple access (TDMA) and code division multiple access (CDMA) allow concurrent access to LEO satellites by a large number of users.
The number of users that can be serviced by a wireless communication system, the system capacity, increases if the power output from each user""s wireless device is decreased to the minimum power needed for quality transmission, and overhead or non-traffic messages or channel activity is reduced. This is the result of decreasing mutual interference between users, which is especially important in limited power environments such as CDMA type communication systems. However, if the power of a user""s signal becomes too low, the quality of service for that user becomes unacceptable. So, there is a desire to maintain as high a power level as possible to have higher quality service.
Thus, the number of users that may be provided service is increased by maintaining overhead power levels and each individual user""s signals at the minimum levels needed for optimum performance. Therefore, the power output of wireless device transmissions are generally controlled using one or more power control methods to minimize interference and maximize communication link quality. Techniques for power control are discussed for example in U.S. Pat. No. 5,383,219, entitled xe2x80x9cFast Forward Link Power Control In A Code Division Multiple Access System,xe2x80x9d issued Jan. 17, 1995; U.S. Pat. No. 5,396,516, entitled xe2x80x9cMethod And System For The Dynamic Modification Of Control Parameters In A Transmitter Power Control System,xe2x80x9d issued Mar. 7, 1995; and U.S. Pat. No. 5,267,262, entitled xe2x80x9cTransmitter Power Control System,xe2x80x9d issued Nov. 30, 1993, which are incorporated herein by reference. In addition, also see U.S. patent application Ser. No. 09/164,384 filed Sep. 30, 1998 entitled xe2x80x9cSystem And Method For Optimized Power Controlxe2x80x9d; and Ser. No. 08/908,528, filed Aug. 7, 1997, entitled xe2x80x9cMethod And Apparatus For Adaptive Closed Loop Power Using Open Loop Measurements,xe2x80x9d which are incorporated herein by reference. The result is the communication system efficiently carries the substantially maximum number of individual user transmissions simultaneously.
Wireless devices, also referred to as user terminals, in current wireless communications systems may be any of several different types. One type is the portable unit, which is a hand held device carried by the user and requires no external power source or antenna system. Another type is the mobile unit or station, which is typically fixed in a vehicle and operates like a desk type phone. A mobile unit has a separate unit (or xe2x80x9cboxxe2x80x9d) that is mounted in the vehicle and contains most of the transmitting and receiving circuits or hardware. A hand held unit such as a phone handset, containing a keypad, speaker and microphone, is connected by one or more cables, conductors, or connectors to the box. A cradle is provided for supporting the handset unit when it is not in operation or is being used in a xe2x80x9chands freexe2x80x9d mode. The box in turn is connected by a cable to an externally mounted xe2x80x9coutdoorxe2x80x9d antenna unit, which transmits and receives signals via a satellite or terrestrial cellular communications system or a base station or gateway.
A third type of user terminal combines the features of both a portable unit and a mobile unit. This type uses a hand held device that can be used as a standalone unit away from the vehicle, and can be connected to a vehicle mounted assembly sometimes called a xe2x80x9ccar kit,xe2x80x9d for use in the vehicle. The car kit uses an external or outdoor unit (ODU) with an outdoor antenna to accommodate communications for the wireless device. A primary advantage of this combination unit or arrangement is that when the wireless device is used in the vehicle it can utilize additional power provided by the vehicle mounted electronics to establish a better and stronger communications link with satellite transceivers. It also allows conservation of internal battery power, drawing on vehicle provided power instead.
Satellite telephone systems are particularly sensitive to outdoor or external antenna unit matching at the mobile unit due to potential path losses and a resulting difficulty in power control. For optimum performance, the power output of the outdoor unit must be calibrated against, or configured with, specific phone transmission circuits in mind. At present, a given car-kit is designed in the factory to function with certain characteristics for phones with which it is to be connected. Once this designing or matching takes place, the autonomous nature of phones and car kits is diminished because the phone is limited to being used with certain car kits or outdoor antenna units, such as specific models or manufacturers, having closely matched characteristics.
This clearly sets certain constraints for mobile units or hand held phones used with car kits in order to provide a closer match between the power output desired by the phone power control systems or methods, and the power actually being delivered by the outdoor unit. Yet, in the actual marketplace it is not uncommon that a user may have more than one portable phone or have one that can be used both as a standalone unit and as a mobile phone when placed in a cradle mounted in a vehicle. Also, a user may commonly upgrade his or her phone as new models come on the market. Therefore, the mobile unit employed with a car kit may change permanently or on a transient basis, creating potential undesirable power matching problems.
One goal of the present invention is to allow a variety of hand held wireless devices to be used with a given vehicle mounted outdoor antenna unit or car-kit, while maintaining a desired level of accuracy for the power output by the outdoor unit. Here, a given outdoor antenna unit is not calibrated against specific hand held device characteristics, but allowed to adjust in response to changing characteristics or power requirements for a device to which it is coupled. For improved or optimum performance, the outdoor antenna unit and the hand held wireless device connected to it need to communicate transmit power levels and requirements.
What is needed is a system and method for the hand held device to inform the outdoor unit of the transmit power level to produce or use in transferring signals, using a control voltage or signal. Further, what is needed is a system and method for allowing the outdoor unit to adjust its transmit power level based on the control voltage.
The present invention is a system and method for controlling transmit power level of an information signal transmitted by an outdoor antenna unit which is used with a cradle or receptacle for coupling to a hand held, or holdable (portable), wireless communication device originating the information signal being controlled. With the present invention, transmit power level is controlled by transmitting a control voltage from a hand held device to the outdoor antenna unit (also referred to as an outdoor unit, or ODU) and then allowing the outdoor unit to adjust its transmit power level via a closed loop circuit. Because the hand held device informs the outdoor unit of the transmit power level to produce, the present invention allows a variety of hand held wireless devices to work with many different outdoor units while maintaining appropriate power output levels.
The system of the present invention includes a logic unit in the hand held device for determining an appropriate transmit power for transmission of the information signal and for producing a control voltage, a closed loop circuit in the outdoor unit for receiving the control voltage from the logic unit and for adjusting the transmit power level of the information signal based on the control voltage using a substantially constant loop gain. The logic unit determines the control voltage based on the data rate for the signal being transmitted.
The closed loop circuit of the present invention includes a power detector for detecting the transmit power level of the outdoor unit, a log amplifier that converts the output of the power detector to a digital format, a control voltage amplifier that determines whether the log amplifier output is equal to the control voltage and then, if necessary, converts the log amplifier output to a voltage format, a gain control amplifier that adjusts the output of the control voltage amplifier and converts it to a digital format, a filter that filters the output of the gain control amplifier, and a power amplifier that converts the output of the filter to a voltage format to produce the new input for the power detector. The present invention also provides for a ROM look-up table that replaces the log amplifier. The closed loop circuit of the present invention maintains a substantially constant loop gain.
The method of the present invention includes detecting the transmit power level of the outdoor unit, determining whether the transmit power level is equal to the control voltage, and adjusting the transmit power level based on the control voltage if the transmit power level is not equal to the control voltage.